To add Facebook (including secret) emoticons to Pidgin, you can use pidgin-EAP, a project which provides emoticon themes for most available protocols, including Facebook. The pack even contains some sound and buddy list themes.

To install pidgin-EAP emoticon themes in Linux, use the following commands in a terminal:

Or, if you want to install it manually, go to the pidgin-EAP GitHub page, click "Download ZIP" on the right, extract the zip and copy its contents to your home directory (note that the zip contains two hidden ".fonts" and ".purple" folders so you'll have to use Ctrl + H in your file manager to see them).